1. Field of the invention
The present invention relates to a delay-breaking fuse preferably employable for protecting an electric motor such as a power window motor for an automobile or the like.
2. Related art
For example, in the case of a load circuit for an electric motor, an excessively high intensity of electric current instantaneously flows along the load circuit just at a point of time when a highest intensity of electric current reaches a certain value enlarged several times at the time of starting of rotation of the motor compared with the value representing an intensity of electric current at the time of a steady state load. In addition, in the case of a power window motor, a motor lock current flows along a load circuit for the motor with a high intensity enlarged several times compared with the intensity of an electric current at the time of a steady state load current of the motor, when window glasses of an automobile are fully closed or opened. Thus, an electric current frequently flows along the load circuit with an intensity in excess of the value representing a steady-state current value even though an abnormality such as a short circuit or the like occurs. In the circumstances as mentioned above, many requests have been raised from users for providing a delay-breaking fuse having excellent operating characteristics which assures that it is not molten and broken on receipt of an electric current simultaneously having a high intensity in excess of the value corresponding to a steady-state electric current, a motor lock current or the like, and moreover, it can reliably shut off an excessively high intensity of electric current at the time of slight short circuit.
With the delay-breaking fuse of the foregoing type, however, when an electric current having an intensity in excess of the value corresponding to a steady-state current such as a motor lock electric current or the like frequently flows along the load circuit, the temperature of an electric conductive melting portion of the fuse is elevated due to repeated feeding of the motor lock current or the like, resulting in a melting/breaking part of the electrical conductive melting portion being molten and broken. Consequently, the fuse can not practically be used for a long time. In other words, running life of the fuse is undesirably shortened.
In view of the foregoing malfunction, a proposal has been made with respect to a cartridge type delay-breaking fuse as shown in FIG. 5 and FIG. 6 wherein a fusible link 51 serving as a delay-breaking fuse includes a electrical conductive melting portion 54 composed of a fuse element 52 and a melting/breaking part 53, and heat radiating plates 56 are connected to female terminal portions 55 in the face-to-face relationship while the heat radiating plates 56 and the female terminal portions 55 are firmly held between a housing 57 and a resilient clamping arm 60 in the clamped state (cf. Unexamined Japanese Utility Model Application No. 62-180852).
The fuse element 52 is received in the housing 57 for the fusible link 51, and it is then fitted onto a pair of male terminals 58 standing upright from., e.g., an electrical connecting box (not shown).
The fuse element 52 is made of an electrical conductive metallic plate to build a substantially U-shaped integral structure. As shown in FIG. 6, each female terminal portion 55 is composed of a wide base plate 59 integrated with the electrical conductive melting portion 54 and a resilient clamping arm 60 of which opposite ends are inwardly bent. The electrical conductive melting portion 54 is bent to exhibit a substantially inverted U-shaped contour having the melting/breaking part 53 located at the central position thereof. With this construction, while the base plate 59 is outwardly oriented, a pair of female terminal portions 55 are located opposite to each other. The heat radiating plate 56 attached to the female terminal portion 55 is made integral with the base plate 59 which is folded on the male terminal 58 inserting side.
Since the fusible link 51 is constructed in the abovedescribed manner, heat radiation can effectively be achieved in the presence of the heat radiating plates 56, although the temperature of the electrical conductive melting portion 54 is elevated due to repeated feeding of the motor lock electric current. Thus, the melting/breaking part 53 is molten and broken with difficult in spite of the repeated feeding of the motor lock current. This leads to the result that durability of the fusible link 51 can substantially be improved.
However, since the fusible link 1 is constructed such that each heat radiating plate 56 is made integral with the base plate 59 by folding the latter, and the thus folded heat radiating plate 56 comes in close contact with the base plate 59 extending from the electrical conductive melting portion 54 while it is located on an electric current circuit extending from one female terminal portion 55 to other female terminal portion 55 via the electrical conductive melting portion 54, there arises a malfunction that the resistance value of the electric current flowing via the electrical conductive melting portion 54 is largely affected by the heat radiating plates 56. Thus, it is practically difficult that the fuse element 52 obtains a desired pre-arching time/current characteristic because the resistance value of the electrical conductive melting portion 54 largely fluctuates.
The fuse element 52 is made to build an integral structure by punching a band-shaped electrical conductive metallic plate. However, since each heat radiating plate 56 extends across the length of the male terminal 58 inserting side of the female terminal portion 55, the whole length of the fuse element 52 to be punched is elongated by a quantity corresponding to twice extensions of the heat radiating plate 56, i.e., the length of two heat radiating plates 56. Thus, there arises a necessity for preparing a band-shaped electrical conductive metallic plate having a wide width, resulting in a production cost required for forming the fuse element 52 being undesirable increased.
When the fuse element 52 is received in the housing 57, each heat radiating plate 56 is held between the inner wall surface of the housing 57 and the base plate 59 in the clamped state, causing the heat radiated from the heat radiating plates 56 to be easily conducted to the inner wall of the housing 57. Thus, there is a possibility that the housing 57 is thermally deformed due the elevated temperature of the housing 57 itself or the color of the housing 57 is changed to other one.